A yacht's mast is supported by shrouds and stays - nautical equivalents of guy wires.

A guy-wire or guy-rope, also known as simply a guy, is a tensioned cable designed to add stability to a free-standing structure. They are used commonly in ship masts, radio masts, wind turbines, utility poles, fire service extension ladders used in church raises and tents. A thin vertical mast supported by guy wires is called a guyed mast. One end of the cable is attached to the structure, and the other is anchored to the ground or some other point at a distance from the structure's base. The tension in the diagonal guy-wire, combined with the compressional strength of the structure, allows the structure to withstand lateral loads such as wind or the weight of cantilevered structures. They are often installed radially, at equal angles about the structure, in trios and quads. This allows the tension of each guy-wire to offset the others. For example, antenna masts are often held up by three guy-wires at 120° angles. Structures with lateral loads, such as electrical utility poles, may require only a single guy-wire to offset the lateral pull of the electrical wires.

Conductive guy cables for radio antenna masts may disturb the radiation pattern of the antenna, so their electrical characteristics must be included in the design.

Utility poles are buried in the ground and have sufficient shear strength to stand on their own; guys are only needed on some poles to support unbalanced lateral loads due to the utility wires attached to them. Guys are particularly needed on dead-end (anchor) poles, where a long straight section of wire line ends, or angles off in another direction. To protect the public against faults that might allow the cable to become electrified, utility guy cables usually have a ceramic strain insulator ("Johnny ball") inserted near the top, to ensure that any dangerous voltages do not reach the lower end of the wire accessible to the public. The lower end where the cable enters the ground is often encased in a length of yellow plastic reflector to make it more visible, so that people or vehicles do not run into it.

In urban areas where the ground area around the pole is restricted, a variation called a sidewalk guy is often used. In this type the guy line extends diagonally from the top of the pole to a horizontal spar brace extending out from the middle of the pole, and from this it continues vertically to the ground. Thus the bottom part of the guy is vertical and does not obstruct headroom, so a sidewalk can pass between the pole and the guy.

An alternative to guy-wires sometimes used on dead-end poles is a push-brace pole, a second pole set at an angle in the ground which butts diagonally against the side of the vertical pole.

Electrical issues complicate the design of guys that support mast antennas. Conductive metal guy-wires longer than one-fifth of the wavelength of the radiated radio waves can act as a resonator, distorting the radiation pattern of the antenna. This also applies to guy wires of neighboring masts situated nearby. To prevent this, each guy wire is divided by strain insulators into multiple sections, each shorter than one-fifth of the wavelength. Cylindrical or egg-shaped porcelain "Johnny ball" insulators are usually used.

The individual sections of the guys can develop large charges of static electricity, especially on very tall masts. The voltage caused by this static electricity can be several times larger than that generated by the transmitter. In order to avoid dangerous and unpredictable discharges, the insulators must be designed to withstand this high voltage, which results at tall masts in over-dimensioned backstage insulators. At each backstage insulator, a lightning arrestor in the form of an arc gap is required for the purpose of over-voltage protection in case of lightning strikes. The insulators and arrestors must be maintained carefully, because an insulator failure can result in a mast collapse.

Cylindrical strain insulator of type used on utility pole and antenna mast guys.

Some newer mast antennas are fitted with insulators at the mast construction and are grounded via coils located near each ground anchor. In some cases it is possible to ground the guys directly at the anchor blocks. This is only possible if the guys do not disturb the radiation pattern of the mast antenna. Guys grounded via a coil or directly have the advantage that maintenance is easier, because all parts requiring maintenance are at the mast and at the anchor basement and that the insulators must only withstand the maximum transmission voltage.

On some mast antennas, guys of non-conductive polymer line are used. Although these guys alleviate the electrical problems associated with conductive cable, they are rarely used as they are not as durable and long-lasting as metal guys.

On antennas for long-wave and VLF, the guys may serve an electrical function, either for capacitive lengthing of the mast or for feeding the mast with the radiation power. In these cases, the guys are fixed without an insulator on the mast, but there is at least one insulator in the guy if necessary. If guys are used for feeding the mast with high frequency power it is often possible to use a grounded mast. The power to the guys is fed via conductor ropes running from the tuning unit to the feed point on the guys.

In ground-anchored guys, the structure which attaches the guy-wire to the ground is called an anchor.[1] The anchor must be adequate to resist the maximum tensile load of the guy wire; both the dead load of the tension of the wire and the maximum possible live load due to wind. Since the guy wire exerts its force at an angle, the anchor has both vertical and lateral (horizontal) forces on it. The anchor relies on the lateral shear strength of the soil to hold it. Several types of anchor are used:

In this type, a hole is excavated and an object with a large surface area is placed in it with the guy wire attached, and the hole is backfilled with earth or concrete.[1] In the traditional form of dead man anchor, a log is buried horizontally in a trench with the guy attached perpendicularly to its center. Modern forms are the plate anchor, in which the guy is attached to a rod with an eyelet extending from the center of a steel plate buried diagonally, perpendicular to the angle of the guy. In the concrete anchor, a diagonal rod with an eyelet extending in the guy direction is cemented into a hole filled with concrete.

This type consists of a rod with wide screw blades on the end and an eyelet on the other for the guy wire.[1] It is screwed deep into the ground, at the same angle as the guy, by a truck-mounted drill machine.

These are used in both soil and rock.[1] A hole is predrilled at the angle of the guy. A steel anchor rod with an eyelet is inserted, and the hole around it is filled with a liquid grout mixture consisting of concrete and an expansion agent. When the grout hardens it expands, holding the rod rigidly.